8

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nection between RESET and SYSOK is optional but highly

recommended for safe operation of the LM3647.

4.3.5 Dimensioning the RCIN circuitry

The RC-circuitry is designed to time the charger so it charges

and regulates at the correct frequency. The values of the R

and C are important, because a change in the RC-value

gives a higher or lower operating frequency which affects

charge quality. The capacitance should be a ceramic type

and preferably an NP0 type, which gives the least frequency

deviation with temperature change.

4.3.6 Dimensioning the DISCHARGE circuitry

The discharge-circuitry controls the discharge rate during the

discharge phase (if used). The discharge output turns on the

transistor Q2, and a current flows from the battery through

the discharge resistor R8. The current through R8 depends

on battery voltage and the value of R8. This value depends

on the maximum discharge rate for the battery-pack. The ap-

proximate value can be calculated using the formula below:

The resistor R7 keeps the transistor Q2 off until the LM3647

has been powered up and is in control of the circuitry.

4.3.7 BUZZER output circuitry

The buzzer-circuitry turns the transistor Q3 on when the

buzzer should sound. If the current consumption for the buzz-

er is lower than 0.3mA then the buzzer may be directly con-

nected to the BUZZER-pin. Please note that the BUZZER-

pin does not generate a PWM-signal, such buzzers must

have their own drive-circuitry. If an electromagnetic buzzer is

used, then the transistor may need a reverse-biased diode to

protect it from harmful voltage spikes.

4.3.8 PWM filter circuitry

The PWM-pin can either output a fast PWM-signal, or a slow

on/off output (for controlling external constant current source,

Ni-Cd/Ni-MH mode only).

Fast PWM-mode:

The RC-network R6, C9 and R5, C1||C2 lowpass-filters the

PWM-signal from the LM3647 to a DC-level that is fed into

the operational amplifier. The resistor R22 is required to pre-

vent DC-output before the LM3647 has control of the RC-net-

works.

R8

MaximumBatteryVoltage

MaximumDisch

eRate

arg

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Q3